Interest in using fuel cells as a clean, alternative power source has driven years of intense research in polymer electrolyte membrane (PEM) fuel cell development to meet the cost and performance targets for automotive and portable applications. Current PEM fuel cells use mainly Nafion® or other perfluorosulfonic acid polymer membranes which have high proton conductivity and good chemical and mechanical stability under fully humidified conditions. However, the widespread use of these membranes has been limited by their cost and poor performance at low relative humidities (RH). Therefore, alternative low-cost membrane materials that have better performance in less humidified conditions are desired.
Polyethersulfones are well-known engineering thermoplastics that have excellent chemical and mechanical properties. Sulfonated versions obtained by post-polymerization sulfonation have been investigated extensively for fuel cell membrane applications. However, poor reproducibility and side reactions in the sulfonation process have led many to explore the direct copolymerization of sulfonated monomers to produce better controlled compositions of sulfonated polyethersulfones. Dispersing hydrophilic fillers into sulfonated polymers has been shown in some cases to enhance proton conductivity at low relative humidities, improve mechanical properties, and decrease methanol permeability (Alberti, G., Casciola, M. Annu. Rev. Mater. Res. (2003) 33, 129-154). McGrath et al. have described addition of heteropolyacids, such as phosphotungstic acid (H3PW12O40), to sulfonated polyethersulfones; however, water solubility of the heteropolyacid leads to leaching of the filler (Kim, Y. S., Wang, F., Hickner, M., Zawodzinski, T. A., McGrath, J. E. J. Membr. Sci. (2003) 212, 263-282; U.S. 2002/0091225).
It would be desirable to prevent leaching of the filler and thereby enhance proton conductivity at low relative humidities, improve mechanical properties, and decrease methanol permeability.